The clinical success of plate and screw systems for internal fixation of fractures is well-documented. Current systems offer the surgeon a choice of conventional plates and screws, locking plates and screws, or various types of combination plates and screws.
Conventional bone plates and screws may be used for treating fractures involving severely comminuted bone or missing bone segments. These conventional systems may also be described as “flexible osteosynthesis” or “biological osteosynthesis” and are particularly well-suited to promoting healing of the fracture by compressing the fracture ends together and drawing the bone into close apposition with other fragments and the bone plate. They are particularly useful in the treatment of comminuted fractures in the diaphyseal region of bones or in regions with severe segmental bone loss. In the case of these fractures, it is imperative to maintain proper bone length while correcting fracture fragments for proper anatomic alignment. With flexible osteosynthesis, the fracture zone is not directly affixed or manipulated, and consequently, the blood circulation in this area is not inhibited.
Bone plates designed for flexible osteosynthesis thus operate similarly to a locking, intramedullary nail, which is anchored only in the metaphyses. Flexible osteosynthesis repair constructs allow for micromotion across the fracture site stimulating callous formation. Since the angular relationships between the plate and screws are not fixed, they can change postoperatively, leading to mal-alignment and poor clinical results.
The primary mechanism for the change in angular relationship is related to energy storage. Threading a bone screw into bone compresses the bone against the plate. The compression results in high strain in the bone, and, consequently, energy storage. With the dynamic loading resulting from physiological conditions, loosening of the plate and screw and loss of the stored energy can result.
Conventional bone screws, i.e. screws that are not secured to a plate so that a fixed angular relationship between the plate and screw is maintained (hereinafter “non-locking screws”) effectively compress bone fragments, but possess a low resistance to shear force that can lead to loosening of the screw.
The development of plates incorporating a fixed angular relationship between the bone plate and screws have been developed to combat this problem. Methods of securing the screw to the plate are known as so-called “locking plates”, “locking screws” or “rigid osteosynthesis”. This type of fixation is particularly useful in treating peri-articular fractures, simple shaft fractures (where nailing is impossible), as well as osteotomies. Aside from the possibility of anatomical repositioning, the bone itself supports and stabilizes the osteosynthesis, which allows for the possibility of putting stress on the extremity earlier and without pain.
Securing the screw in a fixed angle to the plate reduces the incidence of loosening. As the relationship between the locking screws and the plate is fixed, locking screws provide a high resistance to shear or torsional forces. However, locking screws have a limited capability to compress bone fragments. Additionally, locking screws hold the construct in such a rigid position that micromotion across the fracture site may be impeded thereby inhibiting callous formation. Though used successfully for certain fractures, rigid osteosynthesis has been shown to promote the occurrence of non-unions at the fracture site.
A locking screw has threading on an outer surface of its head that mates with corresponding threading on the surface of a plate hole to lock the screw to the plate. Bone plates having threaded holes for accommodating locking screws are known. For example, German Patent Application No. 43 43 117 discloses a bone plate with threaded holes for locking screws. Locking screws have a high resistance to shear force that ensure stability at the bone screw/plate hole interface, but possess a limited ability to compress bone fragments.
Since fractures cannot always be treated with both types of osteosynthesis at the same fixation point, surgeons must frequently compromise because bone plate screw holes only allow him to choose between one of these two types of continuous osteosynthesis discussed above. The ideal fracture stabilization construct would allow the surgeon to choose between continuous flexible osteosynthesis, continuous rigid osteosynthesis and temporary rigid osteosynthesis transforming to flexible osteosynthesis within a pre-defined time period.
By having the option to rigidly fix a fracture fragment via a known location for a pre-determined period of time and allowing that rigid fixation to transform into a region of flexible osteosynthesis, the surgeon is thus enabled to expose the fracture site to a period of stability followed by controlled micromotion thus stimulating bony healing.